Owing to their advantages such as having a low radiation level, a light weight, a thin profile and low power consumption, liquid crystal display (LCD) devices have found wide applications in mobile phones, personal digital assistants (PDAs), notebook computers, personal computers (PCs), TV sets and the like. Because liquid crystal molecules in an LCD panel do not emit light by themselves, an area light source device (e.g., a backlight module) must be provided for the LCD panel in order to accomplish the displaying function. The area light source device is mainly used to provide an area light source with sufficient and uniformly distributed luminance for the LCD panel.
In order to satisfy the requirements on energy consumption, durability and environmental protection, high-performance light emitting diodes (LEDs) have been used in backlight modules to gradually replace the conventional cold cathode fluorescent lamps (CCFLs) as a primary choice. Currently, there are mainly two types of backlight modules that adopt LEDs, the direct-lit type and the edge-lit type. Backlight modules of the direct-lit type feature a simple structure, high luminance, a desirable heat dissipation effect and a small frame, but have the main shortcomings that they require use of a large number of LEDs, have a high cost and tend to present non-uniform luminance and colors and, moreover, make the whole backlight module thick and bulky.
As compared with the backlight modules of the direct-lit type, backlight modules of the edge-lit type overcome the aforesaid shortcomings, but also have shortcomings that the frame is relatively wide and LED lamp shadows or muras can be easily seen at the side edges. Currently in the mainstream edge-lit backlight module structure, LEDs are soldered onto a printed circuit board (PCB) to form a light bar disposed along a side edge of a light guide plate (LGP) and are then fixed to an aluminum extruded piece for heat dissipation. This structure has the following main problems.
Firstly, because the LEDs are disposed at the side edge of the light guide plate, collisions between the LEDs and the light guide plate occur easily. Particularly when the backlight module is handled or impacted, accidental collisions between the LEDs and the light guide plate may cause damage to the LEDs.
Secondly, the thicknesses of the LEDs, the PCB and the aluminum extruded piece add to each other at the side edge of the light guide plate in a direction perpendicular to a side surface of the light guide plate, which makes the whole thickness of the backlight module of the edge-lit type relatively large. Consequently, a wide frame must be used to accommodate the increased thickness, and this is in contradiction to the development tendency towards a narrow frame.
Thirdly, heat generated by the LEDs during operation must be timely dissipated outwards. The heat dissipation path for the LEDs passes through the PCB, a side edge of the aluminum extruded piece and a bottom surface of the aluminum extruded piece to the backplate. Such a long heat dissipation path leads to low heat dissipation efficiency, and the heat that cannot be dissipated timely may have an adverse effect on other elements of the LCD device or even cause degradation in the displaying quality.